J. Phys. Chem. A 2008, 112, 7939–7946
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Synthesis, Spectroscopic Characterization, and Conformational Properties of Trichloromethanesulfenyl Acetate, CCl3SOC(O)CH3 Marlene Cuaquira Reina,†,# Roland Boese,3 Maofa Ge,⊥ Sonia E. Ulic,†,‡ Helmut Beckers,+ Helge Willner,+ and Carlos O. Della Ve´dova*,†,§ CEQUINOR (CONICET-UNLP), Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, 47 esq. 115, (1900) La Plata, Buenos Aires, Repu´blica Argentina, Departamento de Ciencias Ba´sicas, UniVersidad Nacional de Luja´n, Rutas 5 y 7 (6700) Luja´n, Buenos Aires, Repu´blica Argentina, Laboratorio de SerVicios a la Industria y al Sistema Cientı´fico (UNLP-CIC-CONICET), Camino Centenario, Gonnet, Buenos Aires, Repu´blica Argentina, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, Fachbereich C-Anorganische Chemie, Bergische UniVersita¨t Wuppertal, Gaussstrasse 20, 47097 Wuppertal, Germany, and Institut fu¨r Anorganische Chemie, UniVersita¨t GH Essen, UniVersita¨tsstrasse 5-7, D-45117 Essen, Germany ReceiVed: April 24, 2008; ReVised Manuscript ReceiVed: May 12, 2008
Trichloromethanesulfenyl acetate, CCl3SOC(O)CH3, belongs to the family of sulfenic esters. This molecule has been characterized by vibrational spectroscopy. The conformational and geometrical properties of this species have been determined by IR and Raman spectroscopy, X-ray diffraction, and quantum chemical calculations. Geometry optimizations of the most stable forms were performed with ab initio (HF, MP2) and density functional theory (B3LYP) methods. According to our data, this compound results in a gauche-syn conformer with C1 symmetry (gauche orientation around the S-O bond and syn orientation of the CdO double bond with respect to the S-O single bond) for the most stable geometry, and trans-syn conformer with Cs symmetry (trans orientation around the S-O bond and syn orientation of the CdO double bond with respect to the S-O single bond) for the second stable conformer (1.1 and 0.53 kcal/mol higher in energy than the most stable C1 form according to the matrix FTIR spectroscopy and MP2/6-31G* level of the theory, respectively). The crystalline solid (monoclinic, P21/n, a ) 8.0152(17) Å, b ) 5.7922(13) Å, c ) 17.429(4) Å, R ) γ ) 90°, β ) 100.341(3)°) consists exclusively of the main form. The geometrical parameters (X-ray diffraction) are dC-Cl ) 1.767(19) Å, dC-S ) 1.797(2) Å, dS-O ) 1.663(14) Å, dCdO ) 1.189(2) Å, dO-C ) 1.389(3) Å, dC-C ) 1.483(3) Å, angles Cl-C-Cl ) 110.3(11)°, Cl-C-S ) 111.8(12)°, C-S-O ) 97.4(8)°, S-O-C ) 116.7(11)°, O-CdO ) 122.8(19)°, OdC-C ) 127.1(2)°, and the main torsion angles are δ(CSOC) ) 105.9(15)° and δ(SOC(O)) ) 7.6(3)°. The geometrical data calculated with B3LYP/631G++(3df,3pd), B3LYP/6-311G++(3df,3pd), B3LYP/aug-cc-pVTZ, and MP2/6-31G* are in good agreement with diffraction data. Introduction One of the most interesting structural properties of disulfides R1SSR2, peroxides R1OOR2, and sulfenic esters R1SOR2 is the dihedral angle around the S-S, O-O, and S-O bonds, respectively. In these families, the p-shape lone pairs of the two chalcogen atoms are perpendicular to each other, and this orientation allows a maximum orbital interaction between these lone pairs and the opposite S-R or O-R σ* antibonding orbital (anomeric effect, lp(S) f σ*(S-R) or σ*(O–R) and anomeric effect, lp(O) f σ*(O–R) or σ*(S–R)).1,2 This is the usual qualitative explanation for the gauche orientation of substituents * To whom correspondence should be addressed. E-mail: carlosdv@ quimica.unlp.edu.ar. C.O.D.V. is a member of the Carrera del Investigador of CONICET, Republic of Argentina. † CEQUINOR, Facultad de Ciencias Exactas, Universidad Nacional de La Plata. ‡ Universidad Nacional de Luja ´ n. § LaSeISiC (UNLP-CIC-CONICET), Departamento de Quı´mica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata. ⊥ Chinese Academy of Sciences. + Bergische Universität Wuppertal. 3 Universita ¨ t GH Essen. # This work is part of the Ph.D. thesis of M.C.R., who is a doctoral fellow of Regional Program DAAD of Chemistry, Republic of Argentina.
in peroxides, disulfides, and sulfenic esters. Thus, the dihedral angle depends on the form of the lone pairs, the extension of the anomeric effect, and the steric demand of the substituents. Thioperoxides of the type R1SOR2, which are formal derivatives of the sulfenic acid HSOH,3 are much less stable than peroxides or disulfides, and there is still little known about their structural properties. The parent compound, HSOH,3 possesses a gauche structure with a dihedral angle (δ(HSOH) ) 91.3°) closer to that in disulfide, HSSH4 (δ(HSSH) ) 90.76(6)°), than that in peroxide, HOOH5 (δ(HOOH) ) 120.0(5)°). In thioperoxides with two formally sp3-hybridized substituents, CF3SOCF3,6 the dihedral angle is larger than 100° (δ(C-S-O-C) ) 105°), and this value is closer to that in disulfide, CF3SSCF37 (δ(C-S-S-C) ) 98.2°), than that in peroxide CF3OOCF3,8 (δ(C-O-O-C) ) 123.3°). Recently, very unexpected conformational properties were determined for two molecules containing S-O bonds, both with one sp3- and one sp2-hybridized substituent, CF3SOC(O)CH39 and CF3SOC(O)CF3.9 IR (matrix) spectra and GED9 result in mixtures of gauche-syn and trans-syn conformers. In addition to the expected gauche form for both compounds (dihedral angles of δ(C-S-O-C) ) 100(4)° and δ(C-S-O-C) ) 101(3)°, respectively) with a synperiplanar orientation of the
10.1021/jp803609m CCC: $40.75 2008 American Chemical Society Published on Web 08/01/2008
7940 J. Phys. Chem. A, Vol. 112, No. 34, 2008 CdO bond relative to the S-O bond, a trans conformation around the S-O bond appears as a second stable form, which is well reproduced by quantum chemical calculations. The calculated potential functions for internal rotation around the S-O bond for CF3SOC(O)CH39 and CF3SOC(O)CF39 possess a minimum for the trans structure at around 180°, in addition to the minimum near 90° for the gauche structure. In the case of the symmetrically substituted trifluoroacetylsulfenyl trifluoroacetate, CF3C(O)SOC(O)CF3,10 the conjugation between the p-shaped lone pairs of sulfur and oxygen with the π-systems of the two substituents (CF3C(O)) could stabilize a planar trans conformation. However, IR (gas), UV, and 13C NMR spectroscopy predict only the existence of the main conformer, the syn-syn. In CF3C(O)SOC(O)CF310 (δ(C-S-O-C) ) 77°) and in FC(O)SOC(O)CF3,11 (δ(C-S-O-C) ) 75(3)°), the dihedral angle around the S-O bond is also smaller than 90°. It is assumed that such small angles are observed in sulfenic esters with two sp2-hybridized substituents (FC(O) or C(O)CF3). Two trends become apparent from this rather limited number of examples; the dihedral angle (δC-S-O-C) increases as the steric requirements of the substituents for R1SOR2 with two sp3-, one sp3-, and one sp2-hybridized substituents are increased and decreases with two sp2-hybridized substituents. Furthermore, bis(trifluoroaceto) disulfide, CF3C(O)OSSOC(O)CF3,12 and diaceto disulfide, CH3C(O)OSSOC(O)CH3,13 were studied by photoelectron spectroscopy (PES) and theoretical calculations. The dihedral angles around the S-O bonds were reported computed at δ(S-S-O-C) ) 91.8° and δ(S-S-O-C) ) 92.3°, respectively. Photoelectron spectroscopy (PES) and quantum chemical calculations were also performed to investigate the first vertical ionization energies of these two compounds, CCl3SOC(O)CH3,14 the title molecule, and CCl3SOC(O)CF3.14 The dihedral angles of both gauche forms, CCl3SOC(O)CH314 (δ(C-S-O-C) ) 107.0°) and CCl3SOC(O)CF314 (δ(C-S-O-C) ) 108.5°), were calculated by the B3LYP/6-311+G(3df) method. As far as we know, this is the only antecedent reported for the species under study. This work is part of a systematic study on sulfur-oxygen compounds to obtain more accurate structural information about S-O single bonds. We report here the X-ray structure of CCl3SOC(O)CH3, its full vibrational characterization, and the geometrical and conformational properties investigated by IR (gas, liquid), Raman (liquid), NMR (1H, 13C), and UV spectroscopy, with the complement of X-ray diffraction at low temperature and supplemented by quantum chemical calculations. Experimental Section Synthesis. Trichloromethanesulfenyl acetate, CCl3SOC(O)CH3, was prepared using reported methods of the literature,15 and its purity was checked by IR and 1H and 13C NMR spectroscopy. Physical Properties and Spectroscopic Characterization. CCl3SOC(O)CH3 is a pale-yellow liquid, with the characteristic overpowering trichloromethanesulfenyl odor. In the liquid or gaseous state, CCl3SOC(O)CH3 is unstable because of its sensitivity to moisture and disproportionation. The colorless solid sample melts at -20 °C, and the vapor pressure is 0.5 torr at 20 °C. The 1H NMR spectrum shows only a singlet located at 1.8 ppm that corresponds to the CH3 group of the molecule. Furthermore, the 13C NMR spectrum shows a singlet signal at 20.1 ppm assigned to the CH3 group and two singlets at 97.5 and 166.5 ppm, corresponding to the carbon atoms in the CCl3 and CdO groups, respectively.
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Figure 1. Schematic representation of the four conformers of CCl3SOC(O)CH3.
TABLE 1: Calculated Relative Energies, Free Energies (kcal mol-1),a and ν(CdO) Vibrational Frequencies (cm-1) for CCl3SOC(O)CH3 corformerb
method of caculation
gauche-syn HF/6-31G* MP2/6-31G* B3LYP/6-31++G(3df,3pd) B3LYP/6-311++G(3df) B3LYP/6-311++G(3df,3pd) B3LYP/aug-cc-pVTZ trans-syn HF/6-31G* MP2/6-31G* B3LYP/6-31++G(3df,3pd) B3LYP/6-311++G(3df) B3LYP/6-311++G(3df,3pd) B3LYP/aug-cc-pVTZ gauche-anti HF/6-31G* MP2/6-31G* B3LYP/6-31++G(3df,3pd) B3LYP/6-311++G(3df) B3LYP/6-311++G(3df,3pd) B3LYP/aug-cc-pVTZ
∆E ∆G° ν(CH3CdO)c 0.00 0.00 0.00 0.00 0.00 0.00 0.26 0.90 0.17 0.15 0.16 0.01 6.41 5.42 3.83 3.77 3.77 3.95
0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.53 -0.07 -0.02 -0.05 -0.34 6.95 5.96 4.47 4.46 4.37 4.61
2066 1874 1847 1852 1851 1841 2029 1827 1791 1795 1795 1787 2078 1870 1856 1861 1861 1850
(307) (155) (228) (227) (228) (231) (335) (170) (256) (257) (258) (258) (423) (213) (340) (342) (343) (339)
a Includes different multiplicities (m ) 1 for trans and m ) 2 for gauche). b See Figure 1. c In parentheses are the calculated IR intensities in km mol-1.
In the mass spectrum of CCl3SOC(O)CH3, the presence of the molecular ion peak is observed as a low-intensity signal at m/z 208. Other fragments of CCl3SOC(O)CH3 were also assigned, resulting in peaks at m/z values of 79 (5, ClCS+), 43 (100, CH3CO+), 28 (7, CO+), and 15 (10, CH3+); the relative abundance is given in parentheses. The UV-visible spectrum of CCl3SOC(O)CH3 exhibits two absorptions at 233 and 283 nm, and the assignment was made by comparison with CF3C(O)SOC(O)CF3. The band at 233 nm corresponds to the π f π* transition, which is originated from the CdO chromophore, and the second UV absorption at 283 nm is due to the S-O group. Instrumentation. Vibrational Spectroscopy. Infrared (gas, liquid) spectra were recorded on a Bruker Vector 22 FTIR instrument with a resolution of 2 cm-1 in the range of 4000-400 cm-1 using a gas cell with an optical path length of 200 mm and Si windows (0.5 mm thick). Raman spectra of the liquid substance were measured on a Bruker RFS 100/S FT Raman spectrometer in the region of 3500-100 cm-1 with 2 cm-1 resolution. Matrix Isolation. CCl3SOC(O)CH3 was placed into a small U trap that was mounted in front of the matrix support (a metal
Trichloromethanesulfenyl Acetate
Figure 2. Calculated potential curves for internal rotation around the O-C (sp2) bond in CCl3SOC(O)CH3 using HF/6-31G* and B3LYP/ 6-31G*.
Figure 3. IR and Raman spectra of liquid CCl3SOC(O)CH3 at room temperature.
mirror) and cooled at -54 °C. A stream of argon gas was passed over the sample in the U trap, and the resulting mixtures of gaseous CCl3SOC(O)CH3 and argon (≈1:1000) were forced through a quartz tube with a nozzle of 1 mm i.d. that was electrically heated over a length of ≈15 mm. The nozzle was held at temperatures of 20, 210, 310, 400, and 450 °C by deposition onto the matrix support, a rhodium-plated copper mirror cooled at ≈15 K by a He continuous flow cryostat, by the continuous deposition technique. Details of the matrix apparatus have been described elsewhere.16 For the photolysis experiments, radiation from a 150 W mercury high-pressure lamp (TQ150, Haereus, Hanau, Germany) with and without a λ ) 280-335 nm cutoff filter (Schott, Mainz, Germany) was used over a period of 5 to 15 min. IR (Ar matrix) spectra of the deposits were recorded on an IFS66v FTIR spectrometer in the reflectance mode with a transfer optic. The interferometer was equipped with a globar source, a KBr beam splitter, and a MCT-500 detector. The spectra were recorded with a resolution of 0.5 cm-1 in the range of 4000-530 cm-1. UV Spectroscopy. UV-visible spectra of the gas phase of CCl3SOC(O)CH3 were recorded using a glass cell (10 cm optical
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Figure 4. IR spectra in the stretching region of the CH3 group for CCl3SOC(O)CH3 isolated in Ar matrixes at 15 K deposited at different temperatures.
Figure 5. IR spectra in the carbonyl stretching region for CCl3SOC(O)CH3 isolated in Ar matrixes at 15 K deposited at different temperatures.
path length) equipped with quartz windows in a Perkin Elmer Lambda EZ210 spectrometer. The measurements were carried out in the spectral region of 200-600 nm. NMR Spectroscopy. For the 1H and 13C NMR measurements, neat samples were flame-sealed in thin-walled 4 mm o.d. tubes and placed into 5 mm NMR tubes. The spectra were recorded at low temperature (-30 °C) with a Bruker AC 250 NMR spectrometer using CD3OD as an external lock and internally referenced to (CH3)4Si. X-ray Diffraction. The crystallization was performed on the diffractometer at 247 K with a miniature zone melting procedure using focused infrared laser radiation.17 The diffraction intensities were measured at low temperatures on a four circle diffractometer. Intensities were collected with graphite-monocromatized Mo KR radiation using the ω-scan technique. The structure was solved by Patterson syntheses and refined by the full-matrix least-squares method on F2 with the SHELXTLPlus program.18
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TABLE 2: Experimental (IR, Raman) and Calculated Vibrational Wavenumbers of CCl3SOC(O)CH3 IR (liquid)a
Raman (liquid)
IR (Ar matrix)
mode
gauche-syn
gauche-syn
gauche-syn
ν1 ν2 ν3 ν4
3574 3025 2992 2939 1796 1749
ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 v14 ν15 ν16 ν17 ν18 ν19 ν20 ν21 ν22 ν23 ν24 ν25 ν26 ν27 ν28 ν29 ν30
vw vw vw vw vs me
1427 w 1366 m 1211 me 1155 vs 1051 w 993 m 946 we 852 s 813 s
3028 2994 2940 1796 1749e 1432 1367
995 817 757
766 738 608 582 457
s s vw w w
1812 1779e 1432 1367 1203e 1156 1045 989 856 815 758 774 745
610 578 460 408 348 309 288 258 225e 187 169e 85
B3LYP/6-31++G(3df,3pd)b gauche-syn
assignmentc
trans-syn
3157 (4) 3112 (1) 3050 (
